Kepler-11c

Lore & Background

Kepler-11c was announced to the public on February 2, 2011, with its findings published in Nature a day later. Its name derives from its host star Kepler-11, named after the Kepler satellite, and the letter 'c' indicates it is the second-closest planet from the star at discovery. The planet's existence was confirmed through observation of an orbital resonance effect between Kepler-11b and Kepler-11c. Follow-up observations were conducted using telescopes including the Hale, C. Donald Shane, MMT, WIYN, Tillinghast, Keck I, Hobby–Eberly, Smith, and Nordic Optical Telescope.

The host star Kepler-11 is located 2,000 light-years away in the constellation Cygnus. It is nearly identical to the Sun in mass (0.95 solar masses), radius (1.1 solar radii), metallicity ([Fe/H] = 0), and effective temperature (5680 K), but is much older at 8 ± 2 billion years. Kepler-11c orbits every 13.02502 days at a distance of 0.106 AU, with an inclination of 89°, making its orbit almost edge-on as seen from Earth. Its equilibrium temperature is 833 K, about three times Earth's average.

Kepler-11c has a mass 13.5 times Earth's and a radius 3.15 times Earth's, giving it a density of 2.3 g/cm³—more than double that of pure water. The Kepler team has said that Kepler-11b and Kepler-11c are probably composed mostly of water with a thin hydrogen and helium atmosphere. The planet's proximity to its star has blown off most of its atmosphere; formation calculations indicate it lost over 50% of its hydrogen envelope by mass after formation. The inner five planets' orbits would fit within Mercury's orbit, while Kepler-11g orbits much farther out.

Reader's Guide

Kepler-11c is significant as part of the first system discovered with more than three transiting planets, demonstrating the Kepler telescope's ability to detect multiple small planets in a single system. The Kepler-11 system holds the record for being the most compact and flattest planetary system discovered, with the inner five planets' orbits fitting within Mercury's orbit. Kepler-11c's density of 2.3 g/cm³, denser than all of the Sun's gas giants but less dense than any rocky planet, supports the interpretation that it is a water-rich world with a thin atmosphere. The orbital resonance between Kepler-11b and Kepler-11c (close to 5:4 ratio) provides evidence for orbital migration during formation. The planet's estimated loss of over 50% of its hydrogen envelope illustrates the effects of photo-evaporation on close-in exoplanets. The system's discovery was announced on February 2, 2011, and published in Nature the following day, marking a milestone in exoplanet science by revealing a multi-planet system with a flat, compact architecture unlike any previously known.

Did You Know?

• Kepler-11c has a density of 2.3 g/cm³, closest to that of the dwarf planet Pluto.
• The Kepler-11 system is the most compact and flattest planetary system discovered.
• The planet likely lost over 50% of its hydrogen envelope after formation due to photo-evaporation.